Polyester resins derived from terephthalic acid and reactive derivatives thereof, such as dimethylene terephthalate, and alkane diols, e.g., of from 1 to 10 carbon atoms, e.g., ethylene glycol, and 1,4-butanediol, as well as related diols, such as 1,4-cyclohexane dimethanol, and mixtures of such resins have been known for some time and have become important constituents in injection moldable compositions. Workpieces molded from such polyester compositions, alone, or combined with reinforcments, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
Accordingly, polyesters have found significant commercial applications. Polyesters are effectively used as an engineering plastic for electrical components, machines, cars, sporting goods, interior decorative goods and the like. For many of these applications it is desirable to employ a polyester resin having a relatively high molecular weight, i.e., having a melt viscosity of above about 600 poise as measured using a Tinium Olsen melt indexer at 250.degree. C., 0.042 inch orifice (ASTM method D-1238); or having an intrinsic viscosity of above about 0.6 decaliters/gram as measured using a 120-130 mg sample of polyester in a 3:2 mixture of phenol/tetrachloroethane and measuring the time of flow with a Ubbelohde capillary viscometer at 25.degree. C.
However, a problem which many plastics operators have experienced with high molecular weight polyesters is the difficulty in processing the resins. In response thereto, a relatively low molecular weight PBT, less than about 600 poise, as measured using a Tinius Olsen melt indexer at 250.degree. C., 0.042 inch orifice (ASTM method D-1238) has been added to the high molecular weight polyester to provide improved flow blends which still retain the excellent properties of the high molecular weight polyester. However, it has been found that while the blends of high molecular weight polyester and low molecular weight PBT initially exhibit a decrease in melt viscosity, the melt viscosity builds over time. See FIG. 1. Such a build up of viscosity of the blends limits the use of low molecular weight PBT in commercial applications. It would therefore represent a notable advance in the state of the art if a more stable high flow blend of a high molecular weight polyester and low molecular weight PBT could be found.
Jacquiss et al., U.S. Pat. No. 4,532,290, teach stabilizing polycarbonate-polyester compositions against undesirable changes in melting point by adding monosodium phosphate and/or monopotassium phosphate to the compositions. Hepp, U.S. Pat. No. 4,687,802, discloses that the arc track rate of PBT resins can be improved by the addition of a metal salt to the PBT resin.
However, none of the prior art teachings suggests a method of preparing a stable high flow blend of a high molecular weight polyester and a low molecular weight PBT. Surprisingly, the present inventor has now found that if certain phosphorus-containing compounds are added to the blends of high molecular weight polyester and low molecular weight PBT, there is achieved a high flow blend having excellent melt stability.